The small sealed radiation source therapy for prostate cancer is principally conducted by inserting a radiation source in which [Iodine-125] which is a radioactive nuclide is hermetically sealed in a capsule made of titanium (hereinafter, simply referred to as a radiation source) into prostate. The radiation source is normally supplied in the state that 5 or 15 radiation sources are packed in a cartridge, and the cartridge C is provided in the state that it is hermetically sealed in a container in a sterile condition. The radiation sources S are packed in the cartridge C so that their axial directions are aligned (the axial directions are parallel with each other) (see FIG. 9).
In the small sealed radiation source therapy, on the assumption that the amount of radioactivity of the radioactive nuclide hermetically sealed in each radiation source is identical, the number of radiation sources to be inserted into prostate and the inserting position thereof are determined depending on the state of prostate cancer of the individual. The number of radiation sources to be inserted at one operation of the small sealed radiation source therapy is about 50 to 150.
However, it is said that the multiple radiation sources undesirably includes a defective whose amount of radioactivity is different from the nominal value of the supplier of the cartridge in the probability of about one in several hundreds of products, or about two in a hundred of products in products of bad quality. For example, a radiation source having little radioactivity, or a radiation source whose amount of radioactivity is larger than the nominal value can be included. When such a defective radiation source is used, such problems arise that an expected therapeutic effect is not obtained due to shortage of exposed dose, or conversely other tissue is influenced due to excessive exposed dose. For this reason, the American Association of Physicists in Medicine (AAPM) recommends to test at least 10%, and 100%, if possible, of the radiation sources to be used at each facility.
Originally, the radiation intensity of all the radiation sources should be measured at each facility where the radiation sources are used, however, in the method for measuring radiation intensity using an ionization chamber (radiation meter) that is generally used at present, it is necessary to measure the amount of radioactivity of the capsule one by one. Since this has the disadvantages (1) to (7) below, it is actually difficult to measure the radiation intensity of all of the radiation sources at each facility.                (1) It is necessary to take out the cartridge that is packaged in sterile condition from the bag.        (2) It is necessary to take out the radiation source from the cartridge.        (3) Since the radiation source is measured one by one, very long time is required.        (4) It is necessary to pack the radiation source taken out from the cartridge again in the cartridge.        (5) It is necessary to sterilize the cartridge into which the radiation source is packed again.        (6) It is difficult to avoid exposure to radiation of the operator's hands and fingers in the operations of (1) to (5).        (7) A special calibrated ionization chamber is required.        
In this respect, a radiation intensity measuring apparatus for each radiation source in the state that the radiation source is packed in the cartridge has been developed (Patent Document 1).
Patent Document 1 discloses a technique regarding a radiation intensity measuring apparatus for a radiation source, and the measuring apparatus has a receiving part for receiving a cartridge in which small sealed radiation sources are packed in its interior, and is also provided with an insertion opening through which the cartridge is inserted into the receiving part from outside, and a plurality of openings penetrating the receiving part and outside.
With such a configuration, a cartridge is inserted into the receiving part of the measuring apparatus through the insertion opening, and the measuring apparatus is positioned on the X-ray film so that the multiple openings are in contact with the X-ray film. Since the radiation emitted from each radiation source leaks outside the measuring apparatus through the corresponding opening, the X-ray film which is in contact with the measuring apparatus is exposed to the leaking radiation, and information of the radiation intensity of each radiation source is recorded on the X-ray film. Therefore, by analyzing the record on the X-ray film, it is possible to obtain desired information.
Since the measuring apparatus of Patent Document 1 measures the radiation source packed in the cartridge as it is, the above problems (2) to (4) can possibly be cleared up, but the above problems (1) and (5) cannot be solved because measurement cannot be executed unless the cartridge packaged in a sterile condition is taken out from the bag.
In the measuring apparatus of Patent Document 1, the problem of decrease in measurement accuracy of radiation intensity arises for the reason as will be mentioned below although the above problems (2) to (4) can be solved.
In the case of the measuring apparatus of Patent Document 1, the measuring apparatus exposes the X-ray film to light by radiation respectively leaking from the multiple openings h. Therefore, for obtaining the information of the radiation intensity of each radiation source S, it is necessary to accurately align the center axes of the multiple radiation sources S and the center axes of the multiple openings h so that one radiation source corresponds to each opening h.
However, not all of the radiation sources S packed in the cartridge C are packed at the same interval, and slight difference arise in arrangement of the radiation sources S between individual cartridges C. For example, the radiation sources S can include a radiation source S having a wire diameter different from the normal wire diameter, and such a radiation source S can be packed. In this case, as illustrated in FIG. 9(C), when the positions of the multiple openings h are formed at regular intervals in accordance with a normal wire diameter of the radiation source S (0.8 mm), the center axis of a certain radiation source S can be deviated from the center axis of the opening h. As a result, a radiation source S for which accurate radiation intensity cannot be measured arises, and measurement accuracy of the radiation intensity deteriorates.
On the other hand, as a technique for solving the problems (1) to (7) of conventional methods, the technique of Patent Document 2 has been developed.
Patent Document 2 discloses a radiation intensity measuring apparatus capable of measuring radiation intensity of a radiation source packed in a cartridge to be measured in the state that the cartridge is accommodated in a bag or a container. This radiation intensity measuring apparatus has holding means for holding a cartridge accommodated in a bag or container, and an accommodating part having an accommodation space into which the cartridge held by the holding means is to be carried. The accommodating part is provided with a slit that communicates between the interior of the accommodation space and outside. For this reason, by carrying the cartridge into the accommodation space of the accommodating part by the holding means, radiation emitted from each radiation source is emitted outside the accommodating part through the slit. Therefore, by measuring the intensity of the radiation emitted outside the accommodating part, it is possible to measure the intensity of the radiation emitted from the radiation source while the cartridge is kept accommodated in a bag or container.
In addition, the slit has a width that is smaller than the axial diameter of the radiation source, so that the holding means can carry the cartridge into the accommodation space while keeping the axial direction of the slit and the axial directions of the radiation sources packed in the cartridge parallel with each other. Therefore, as the radiation sources packed in the cartridge are sequentially passed through the position of the slit, the intensity of radiation emitted outside the accommodating part through the slit varies with movement of the radiation sources. Therefore, by measuring the variation in radiation intensity, it is possible to calculate the intensity of radiation emitted from each radiation source based on the variation in the radiation intensity.